Arsenal boss Arsene Wenger has lamented over Diego Costa costing Arsenal their last two games in an interview with the BBC on Sunday.
“Costa has got two of our players sent off in the last two games against Chelsea,” said Wenger.
“Is the decision right or wrong? I don’t know. That is a fact, without accusing him of anything.”
Mertesacker was sent off in the 18th minute after tripping Costa – who scored the only goal in Chelsea’s win.
Costa was also involved in the clash that saw Arsenal’s Gabriel sent off in Mertesacker’s 18th-minute dismissal changed the complexion of Sunday’s game at the Emirates, with Arsenal’s defeat leaving them three points behind Premier League leaders Leicester.
It is the seventh red card the Gunners have been shown in games against Chelsea – their most against any one team – and the third they have suffered this season against the Blues, with Santi Cazorla also sent off at Stamford Bridge.
Costa was given a retrospective three-game ban after that game for a clash with Laurent Koscielny.
Wenger added: “I don’t know whether Costa was offside or not or whether Per Mertesacker touched him or not. It was a decision we had to take on the chin and I thought we did that.
“The referee was very quick to take the red card out. We coped with the decision well and should have had a draw at least. We had the chances. Mentally we were focused. Sometimes when you lose you can still say ‘well done’ to the players.”
Asked whether Costa had made the most of the situation, Wenger said: “Yes. That is the game of the striker. Diego Costa is good at that.”
Chelsea boss Guus Hiddink, predictably, disagreed and said: “There was no doubt. It was a great ball through and Diego could run straight at Petr Cech but he was tripped. That is obvious.”